1. Field of the Invention
The present invention relates to an AC-DC converter of a type having an improved power factor to make load acting on components of a circuit thereof to be constant regardless of the level of supplied voltage.
2. Technical Background
A rectifying and smoothing circuit of a capacitor input type has been used for rectifying an AC input to be received by an AC-DC converter of a variety of electronic units. However, there arises a problem now in that harmonic distortion, which is generated in a commercial power supply line by an electronic unit that includes the rectifying and smoothing circuit of the capacitor input type, disorder the operations of other electronic units.
Accordingly, there arises a desire of obtaining an AC-DC converter of a type having an improved power factor that is free from the generation of the harmonic distortion. Therefore, AC-DC converters adapted to a variety of power factor improvement methods have been disclosed.
FIG. 4 illustrates an example of a circuit of a conventional AC-DC converter of a type having an improved power factor. The AC-DC converter shown in FIG. 4 is arranged in such a manner that an AC input terminal of a rectifying circuit 1 is connected to a commercial power supply line while interposing input terminals 9A and 9B, a DC output terminal of the rectifying circuit 1 is connected to an input terminal of a booster converter circuit 2b, an output terminal of the booster converter circuit 2b is connected to an input terminal of a DC-DC converter circuit 3, and an output terminal of the DC-DC converter circuit 3 is connected to an external load while interposing output terminals 10A and 10B.
The booster converter circuit 2b of the AC-DC converter shown in FIG. 4 has a circuit structured as follows.
A series circuit comprising a choke coil L1 and a diode D1 is connected to a position between an input terminal and an output terminal of the high potential side of the booster converter circuit 2b, the diode D1 making a direction from the choke coil L1 toward the output terminal to be a forward direction.
A main electric current passage of a switching transistor Q1 is connected to a position between the anode of the diode D1 and a low-potential-side line of the booster converter circuit 7.
An output capacitor C1 is connected to a position between the cathode of the diode D1 and a low-potential-side line.
A series circuit comprising resistors R3 and R4 is, in parallel, connected to the output capacitor C1.
A booster-converter control circuit 7 comprising an error amplifier 4, a pulse-width modulation circuit 5, a reference voltage circuit 6, resistors R1 and R2 is connected to a position between a voltage dividing point of the resistors R3 and R4 and the gate of the switching transistor Q1 so that the terminal voltage of the output capacitor C1 is controlled.
The booster-converter control circuit 7 is structured as follows.
A series circuit comprising the resistors R1 and R2 is connected to a position between an output terminal of the reference voltage circuit 6 and the low-potential-side line. A voltage dividing point of the resistors R1 and R2 is connected to a negative-side input terminal of the error amplifier 4.
A voltage dividing point of the resistors R3 and R4 is connected to a positive-side input terminal of the error amplifier 4.
An output terminal of the error amplifier 4 is connected to the pulse-width modulation circuit 5, and an output terminal of the pulse-width modulation circuit 5 is connected to the gate of the switching transistor Q1.
The waveform of input and output voltage to and from the booster converter circuit 2b structured as described above is as shown in FIG. 5.
Referring to FIG. 5, VAC1, VAC2 and VAC3 are AC voltages having different levels, the AC. voltages VAC1, VAC2 and VAC3 being received by the AC-DC converter.
VI1, VI2 and VI3 are voltages obtained by rectifying the AC voltages VAC1, VAC2 and VAC3 by the rectifying circuit 1, the voltage VI1, VI2 and VI3 being received by the booster converter circuit 2b.
V0 is voltage transmitted from the booster converter circuit 2b, the voltage V0 being, by the operation of the booster-converter control circuit 7, constant with respect to the supplied voltage.
AC voltage to be supplied to the AC-DC converter varies from 100 V to 240 V over the world. In order to be adaptable to the various AC voltage levels, the terminal voltage of the output capacitor C1 of the booster converter circuit 2b has generally been controlled to a level higher than the rectification peak level of the highest AC voltage by tens of volts, for example, to about 380 V.
When a consideration is made that the quantity of electric power to be processed by the booster converter circuit 2b is divided into converted power to be stored and simple transmission power, the converted power to be stored is expressed by the following equation: EQU P=(V0-V1)I0 (1)
where P is converted power to be stored, V0 is output voltage, V1 is supplied voltage and I0 is transmitted electric current.
As can be understood from Equation (1), the converted power P to be stored is enlarged in proportion to the difference between the supplied voltage V1 and the output voltage V0. The enlargement of the converted power P to be stored makes the load to be borne by the components of the booster converter circuit 2b to be heavier. As a result, the size of each element must be increased and the heating value is inevitably enlarged.
When a circuit is designed, the terminal voltage of the output capacitor C1 must be set to a level which is, by tens of volts, higher than the peak value of rectification of the highest AC voltage and circuit elements must be selected while assuming that the circuit is driven with the lowest AC voltage. Therefore, a problem arises in that the size and cost of an AC-DC converter of the type having the improved power factor cannot be reduced.
It should be noted that the voltage V1 to be supplied to the booster converter circuit 2b is treated as DC voltage for easy understanding although it is in a pulsating waveform obtained by rectifying the AC supplied voltage. The booster converter circuit 2b is also called a power-factor-improved active filter which performs a power factor improving operation, the description of which is omitted here because it has been described in detail in known documents. Further, a triangle wave oscillation circuit to be connected to the pulse-width modulation circuit 5 is omitted from FIG. 5.
The foregoing facts are also applied to the description of the embodiments of the present invention to be made later.